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Wednesday, September 18, 2013

How Birds Got Their Wings: Fossil Data Show Scaling of Limbs Altered as Birds Originated from Dinosaurs

Archaeopteryx lithographica, specimen displayed at the Museum für
Naturkunde in Berlin. Believed to be a transitional species between
theropod dinosaurs and birds, Archaeopteryx had longer forelimbs and
shorter hind limbs than its ancestors. (Credit: By H. Raab (User:Vesta)
(Own work) [CC-BY-SA-3.0 or GFDL], via Wikimedia Commons)

Sep. 17, 2013 — Birds
originated from a group of small, meat-eating theropod dinosaurs called
maniraptorans sometime around 150 million years ago. Recent findings
from around the world show that many maniraptorans were very bird-like,
with feathers, hollow bones, small body sizes and high metabolic rates.

But the question remains, at what point did forelimbs evolve into wings -- making it possible to fly?
McGill University professor Hans Larsson and a former graduate
student, Alexander Dececchi, set out to answer that question by
examining fossil data, greatly expanded in recent years, from the period
marking the origin of birds.

In a study published in the September issue of Evolution,
Larsson and Dececchi find that throughout most of the history of
carnivorous dinosaurs, limb lengths showed a relatively stable scaling
relationship to body size. This is despite a 5000-fold difference in
mass between Tyrannosaurus rex and the smallest feathered
theropods from China. This limb scaling changed, however, at the origin
of birds, when both the forelimbs and hind limbs underwent a dramatic
decoupling from body size. This change may have been critical in
allowing early birds to evolve flight, and then to exploit the forest
canopy, the authors conclude.

As forelimbs lengthened, they became long enough to serve as an
airfoil, allowing for the evolution of powered flight. When coupled with
the shrinking of the hind limbs, this helped refine flight control and
efficiency in early birds. Shorter legs would have aided in reducing
drag during flight -- the reason modern birds tuck their legs as they
fly -- and also in perching and moving about on small branches in trees.
This combination of better wings with more compact legs would have been
critical for the survival of birds in a time when another group of
flying reptiles, the pterosaurs, dominated the skies and competed for
food.

"Our findings suggest that birds underwent an abrupt change in their
developmental mechanisms, such that their forelimbs and hind limbs
became subject to different length controls," says Larsson, Canada
Research Chair in Macroevolution at McGill's Redpath Museum. Deviations
from the rules of how an animal's limbs scale with changes in body size
-- another example is the relatively long legs and short arms of humans
-- usually indicate some major shift in function or behaviour. "This
decoupling may be fundamental to the success of birds, the most diverse
class of land vertebrates on Earth today."

"The origin of birds and powered flight is a classic major
evolutionary transition," says Dececchi, now a postdoctoral researcher
at the University of South Dakota. "Our findings suggest that the limb
lengths of birds had to be dissociated from general body size before
they could radiate so successfully. It may be that this fact is what
allowed them to become more than just another lineage of maniraptorans
and led them to expand to the wide range of limb shapes and sizes
present in today's birds."

"This work, coupled with our previous findings that the ancestors of
birds were not tree dwellers, does much to illuminate the ecology of
bird antecedents." says Dr. Dececchi. "Knowing where birds came from,
and how they got to where they are now, is crucial for understanding how
the modern world came to look the way it is."

Funding for the research was provided by the Fonds de recherche du
Québec -- Nature et technologies, the Canada Research Chairs program,
and the National Sciences and Engineering Research Council of Canada.

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The above story is based on materials provided by McGill University. Note: Materials may be edited for content and length. For further information, please contact the source cited above.